Forming iron nitride hard magnetic materials using chemical vapor deposition or liquid phase epitaxy

ABSTRACT

A hard magnetic material includes α″ Fe 16 N 2 . In some examples, the hard magnetic material may be formed by a technique utilizing chemical vapor deposition or liquid phase epitaxy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage entry under 35 U.S.C. § 371 of International Application No. PCT/US2015/043812, filed Aug. 5, 2015, which claims the benefit of U.S. Provisional Patent Application No. 62/035,245, filed Aug. 8, 2014. The entire contents of International Application No. PCT/US2015/043812 and U.S. Provisional Patent Application No. 62/035,245 are incorporated herein by reference.

This invention was made with government support under DE-AR0000199 awarded by the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

The disclosure relates to hard magnetic materials and techniques for forming hard magnetic materials.

BACKGROUND

Permanent magnets play a role in many electromechanical systems, including, for example, alternative energy systems. For example, permanent magnets are used in electric motors or generators, which may be used in vehicles, wind turbines, and other alternative energy mechanisms. Many permanent magnets in current use include rare earth elements, such as neodymium, which result in high magnetic energy product. These rare earth elements are in relatively short supply, and may face increased prices and/or supply shortages in the future. Additionally, some permanent magnets that include rare earth elements are expensive to produce. For example, fabrication of NdFeB and ferrite magnets generally includes crushing material, compressing the material, and sintering at temperatures over 1000° C., all of which contribute to high manufacturing costs of the magnets. Additionally, the mining of rare earth can lead to severe environmental deterioration.

SUMMARY

The disclosure describes hard magnetic materials including α″-Fe₁₆N₂ and techniques for forming hard magnetic materials including α″-Fe₁₆N₂ using chemical vapor deposition (CVD) or liquid phase epitaxy (LPE). Hard magnetic materials including α″-Fe₁₆N₂ may provide an alternative to permanent magnets that include a rare earth element, as Fe₁₆N₂ has high saturation magnetization, high magnetic anisotropy constant, and high magnetic energy product.

In some examples, the disclosure describes a method including heating an iron source to form a vapor comprising iron-containing compound; depositing iron from the vapor comprising the iron-containing compound and nitrogen from a vapor comprising a nitrogen-containing compound on a substrate to form a layer comprising iron and nitrogen; and annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂.

In some examples, the disclosure describes a method including submerging a substrate in a coating solution comprising a nitrogen-containing solvent and an iron source. The coating solution may be saturated with the iron source at a first temperature above a liquidus temperature of an iron-nitrogen mixture to be deposited from the coating solution. The method further may include submerging a substrate in the coating solution and cooling the coating solution to a second temperature to form a supersaturated coating solution. The second temperature may be below the liquidus temperature of the iron-nitrogen mixture. The method additionally may include keeping the substrate in the supersaturated coating solution to allow a coating comprising iron and nitrogen to form on the substrate, and annealing the coating comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂.

In some examples, the disclosure describes an article comprising a substrate and a layer comprising α″-Fe₁₆N₂ on the substrate, wherein the layer was formed using at least one of CVD or LPE.

In some examples, the disclosure describes a system for performing at least one of CVD or LPE to form an article comprising a substrate and a layer comprising α″-Fe₁₆N₂ on the substrate.

In some examples, the disclosure describes a workpiece comprising at least one phase domain including α″-Fe₁₆N₂, wherein the at least one phase domain was formed using at least one of CVD or LPE.

In some examples, the disclosure describes an article comprising a plurality of workpieces. At least one workpiece of the plurality of workpieces includes at least one phase domain including α″-Fe₁₆N₂, and the at least one phase domain was formed using at least one of CVD or LPE.

The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

The summary, as well as the following detailed description, is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, there are shown in the drawings examples; however, the disclosure is not limited to the specific techniques, compositions, and devices disclosed. In addition, the drawings are not necessarily drawn to scale.

FIG. 1 is a conceptual and schematic diagram illustrating an example chemical vapor deposition system for forming a hard magnetic material including α″-Fe₁₆N₂.

FIG. 2 is an iron nitride phase diagram.

FIG. 3 is a conceptual diagram that shows an α″-Fe₁₆N₂ unit cell.

FIG. 4 is a conceptual diagram illustrating a material having iron or iron nitride (e.g., Fe₈N) domains and α″-Fe₁₆N₂ domains.

FIG. 5 is a conceptual and schematic diagram illustrating an example chemical vapor deposition system for forming a hard magnetic material including α″-Fe₁₆N₂.

FIG. 6 is a conceptual and schematic diagram illustrating an example system for forming a coating including α″-Fe₁₆N₂ on a substrate using LPE.

FIG. 7 is an iron-nitrogen solid-liquid phase diagram.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this disclosure is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular examples and is not intended to be limiting of the claims. When a range of values is expressed, another example includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another example. All ranges are inclusive and combinable. Further, a reference to values stated in a range includes each and every value within that range. As used herein, use of the term “comprising” should also support other embodiments utilizing the terms “consisting of” and “consisting essentially of.”

It is to be appreciated that certain features of the disclosure which are, for clarity, described herein in the context of separate examples, may also be provided in combination in a single example. Conversely, various features of the disclosure that are, for brevity, described in the context of a single example, may also be provided separately or in any subcombination.

The disclosure describes hard magnetic materials including α″-Fe₁₆N₂ and techniques for forming hard magnetic materials including α″-Fe₁₆N₂ using chemical vapor deposition (CVD) or liquid phase epitaxy (LPE). Hard magnetic materials including α″-Fe₁₆N₂ may provide an alternative to permanent magnets that include a rare earth element, as Fe₁₆N₂ has high saturation magnetization, high magnetic anisotropy constant, and high magnetic energy product.

In some examples, the hard magnetic materials including α″-Fe₁₆N₂ may be in the form of a workpiece comprising at least one phase domain comprising α″-Fe₁₆N₂. The workpiece may include, for example, a pellet, a rod, a thin film, a nanoparticle, a powder, or a nanoscale powder. In some examples, the hard magnetic materials including α″-Fe₁₆N₂ may be in the form of an article including a plurality of workpieces. At least one workpiece of the plurality of workpieces may include at least one phase domain comprising α″-Fe₁₆N₂. The article may include, for example, an electric motor, a generator, a sensor, an actuator, a component of an automotive vehicle, or a component of a wind turbine.

The techniques for forming hard magnetic materials including α″-Fe₁₆N₂ may include CVD or LPE. Either of these techniques may be used to deposit a thin film including at least one layer including α″-Fe₁₆N₂ on a substrate. In some examples, the substrate may include a semiconductor, such as silicon, GaAs, InGaAs, or the like. In other examples, the substrate may include another material, such as a glass, a high temperature polymer, SiC, MgO, SiO₂ (e.g., a layer of SiO₂ on a Si or other semiconductor substrate), SiN, SiAlC, TiN, or the like.

Chemical vapor deposition may allow incorporation of hard magnetic materials including α″-Fe₁₆N₂ into semiconductor devices and incorporation of forming of α″-Fe₁₆N₂ into semiconductor processes. For example, hard magnetic materials including α″-Fe₁₆N₂ may be incorporated into magnetic random access memory (MRAM), magnetic logic devices, magnetic storage devices, magnetic microelectromechanical systems (MEMS), micro motors, micro actuators, nano motors, nano actuators, or the like.

FIG. 1 is a conceptual and schematic diagram illustrating an example chemical vapor deposition system 10 for forming a hard magnetic material including α″-Fe₁₆N₂. System 10 includes a chemical vapor deposition (CVD) chamber 12, which may enclose a susceptor 14. A substrate 16 is held by susceptor 14, and a coating 18 is formed on at least a portion of substrate 16. CVD chamber 12 may include, for example, quartz or another refractory material. In some examples, CVD chamber 12 may be formed of a material that is substantially transparent to radio frequency (RF) magnetic energy.

In some examples, CVD chamber 12 is at least partially surrounded by RF induction coils 20. RF induction coils 20 may be electrically connected to an RF source (not shown in FIG. 1), which causes an alternating electrical current at RF to flow through RF induction coils 20. In some examples, the RF magnetic field generated by RF induction coils 20 may be absorbed by susceptor 14, which converts the RF energy to heat. This heats substrate 16. Hence, in some examples, susceptor 14 may include graphite or another material that absorbs RF energy of the frequency generated by RF induction coils 20.

In some examples, susceptor 14 may be shaped or oriented to position substrate 16 at an incline with respect to inlet 22. Positioning substrate 16 at an incline with respect to inlet 22 may reduce or substantially eliminate downstream depletion, which is a phenomena in which downstream portions of substrate 16 are coated with a thinner coating than upstream portions of substrate 16 due to depletion of reactants from the coating gas as the coating gas flows along a substantially horizontal substrate 16.

In some examples, rather than including a susceptor 14 heated by RF induction coils 20, CVD chamber 12 may be heated such that an entire volume of CVD chamber 12 is heated. For example, CVD chamber 12 may be disposed in a furnace, or CVD chamber 12 may be formed of a material that absorbs RF energy and heats the volume of CVD chamber 12.

Substrate 16 may include any material on which coating 18 may be formed. In some examples, substrate 16 may include a semiconductor, such as silicon, GaAs, InGaAs, or the like. In other examples, the substrate may include another material, such as a glass, a high temperature polymer, SiC, MgO, SiO₂ (e.g., a layer of SiO₂ on a Si or other semiconductor substrate), SiN, SiAlC, TiN, or the like.

In some examples, substrate 16 may include a crystalline material with a different lattice structure, different lattice parameters, or both, than α″-Fe₁₆N₂. In some examples, substrate 16 additionally or alternatively may have a different coefficient of thermal expansion (CTE) than α″-Fe₁₆N₂. In examples in which substrate 16 includes at least one of a different lattice structure, different lattice parameters, or a different CTE than α″-Fe₁₆N₂, substrate 16 may exert a strain on layer 18 during an annealing technique, which may facilitate formation of α″-Fe₁₆N₂ in coating 18.

CVD chamber 12 may include an inlet 22 and an outlet 24. Inlet 22 may be fluidically connected to one or more sources of coating gases. For example, in system 10, inlet 22 is fluidically connected to a carrier gas source 26, a first source 30 of a coating constituent, and a second source 34 of coating constituent.

In some examples, carrier gas source 26 may include a gas that carries the coating gas to the interior of CVD chamber 12. In some examples, carrier gas source 26 may include a source of substantially inert gas (e.g., a gas that is substantially non-reactive with other elements and compounds present in system 10 during operation of system 10). A substantially inert gas may include, for example, a noble gas, such as argon.

In some examples, carrier gas source 26 additionally or alternatively may include a gas that may react with one or more elements and compounds present in system 10. For examples, carrier gas source 26 may include a source of hydrogen gas (H₂). In some examples, hydrogen gas may react with an iron precursor to liberate iron. In some instances, carrier gas source 26 may include a mixture of a substantially inert gas and a gas that reacts with one or more elements and compounds present in system 10. For example, carrier gas source 26 may include a mixture of hydrogen gas and argon.

Carrier gas source 26 may be fluidically connected to CVD chamber 12 via conduit or piping, and at least one valve 28. Valve 28 may be used to control flow of carrier gas from carrier gas source 26 to CVD chamber 12.

System 10 also includes first source 30. First source 30 may include a source of a vapor including a nitrogen-containing compound. In some examples, first source 30 may include a gaseous source of a nitrogen precursor, such as gaseous ammonia (NH₃). In other examples, first source 30 may include a liquid or solid source of a nitrogen precursor, such as ammonium nitrate (NH₄NO₃; solid), an amide (liquid or solid), or hydrazine (liquid).

Amides include a C—N—H bond and hydrazine includes an N—N bond. Ammonium nitrate, amides and hydrazine may serve as a nitrogen donor for forming the powder including iron nitride. Example amides include carbamide ((NH₂)₂CO; also referred to as urea), methanamide (Formula 1), benzamide (Formula 2), and acetamide (Formula 3), although any amide may be used.

In some examples, amides may be derived from carboxylic acids by replacing the hydroxyl group of a carboxylic acid with an amine group. Amides of this type may be referred to as acid amides.

In examples in which the nitrogen-containing compound in first source 30 is a solid or liquid, first source 30 may include a heat source to vaporize the nitrogen-containing compound and form a vapor including a nitrogen-containing compound.

First source 30 may be fluidically connected to CVD chamber 12 via conduit or piping, and at least one valve 32. Valve 32 may be used to control flow of nitrogen-containing vapor from first source 30 to CVD chamber 12.

System 10 also includes second source 34. Second source 34 may include a source of iron or an iron precursor (or donor). In the example shown in FIG. 1, second source 34 contains a liquid iron donor 36, such as FeCl₃ or Fe(CO)₅. Second source 34 is fluidically coupled to a gas source 38 via valve 40, which controls flow of gas source 38 into second source 34. In some examples, gas source 38 may be a source of hydrogen (H₂) gas or another reducing gas.

Gas from gas source 38 flows into second source 34 and vaporizes at least some of liquid iron donor 36. Gas from gas source 38 then carries the vapor including the iron-containing compound into CVD chamber 12 through inlet 22.

Valves 28, 32, and 40 may be used to control the total flow rate of gases and vapors into CVD chamber 12, and the relative proportion of carrier gas, the vapor including the nitrogen-containing compound, and the vapor including the iron-containing compound in the gases and vapors flowing into CVD chamber 12. In some examples, valves 28, 32, and 40 may be controlled to produce an atomic ratio of iron to nitrogen in the gases and vapors flowing into CVD chamber 12 to be between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen). For example, the atomic ratio between iron and nitrogen atoms in the gases and vapors flowing into CVD chamber 12 may be about 9:1 (iron:nitrogen), about 8:1 (iron:nitrogen), or about 6.65:1 (iron:nitrogen).

In some examples, the flow rate of the carrier gas may be between about 5 standard cm³/minute (sccm) and about 5,000 sccm, the flow rate of the vapor including the nitrogen-containing compound may be between about 10 sccm and about 1,000 sccm, and the flow rate of the vapor including the iron-containing compound may be between about 100 sccm and about 5,000 sccm. Flow rates such as these may result in a growth rate of coating 18 of between about 100 micrometers per hour (μm/h) and about 1,000 μm/h.

In some examples, substrate 16 may be heated by susceptor 14 and RF induction coils 20 above a decomposition temperature of the iron-containing compound, the decomposition temperature of the nitrogen-containing compound, or both. For example, substrate 16 may be heated to a temperature between about 200° C. and about 1,000° C. by susceptor 14 and RF induction coils 20.

In some examples in which substantially only susceptor 14 and substrate 16 is heated, the iron-containing compound and the nitrogen-containing compound may decompose to release iron and nitrogen, or may react with each other to form an iron nitride compound. Because substrate 16 is heated, this reaction or reactions may occur at the surface of substrate 16, resulting in coating 18 being formed and including iron and nitrogen.

In examples in which substantially the entire volume of CVD chamber 12 is heated (e.g., by a furnace), the decomposition reactions or reaction between the iron-containing compound and the nitrogen-containing compound may occur above substrate within the volume of CVD chamber 12. The liberated iron and nitrogen atoms or iron nitride compound then may deposit on the surface of substrate 16 in coating 18.

In some examples, a reaction between the iron-containing compound and the nitrogen containing compound may include: 16FeCl₃+2NH₃+21H₂→2Fe₈N+48HCl

In some examples, portions of substrate 16 may be masked, leaving only portions of substrate 16 exposed for coating 18 to be formed. In other examples, portions of coating 18 may be etched after deposition of coating 18 to remove the portions of coating 18, leaving only portions of substrate 16 coated with coating 18. In this way, coating 18 may be controllably formed on only selected portions of substrate 16.

As described above, the ratio of iron to nitrogen in the gases and vapors entering CVD chamber 12 may be between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen), such as about 8:1 (iron:nitrogen). Coating 18 may include approximately the same ratio of iron to nitrogen in the gases and vapors entering CVD chamber 12. Thus, coating 18 may include an iron to nitrogen ratio of between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen), such as about 9:1 (iron:nitrogen), about 8:1 (iron:nitrogen), or about 6.65:1 (iron:nitrogen).

In some examples, coating 18, as deposited, may include at least one type of iron nitride, such as, for example, FeN, Fe₂N (e.g., ξ-Fe₂N), Fe₃N (e.g., ξ-Fe₃N), Fe₄N (e.g., γ′-Fe₄N, γ-Fe₄N, or both), Fe₂N₆, Fe₈N, α″-Fe₁₆N₂, or FeN_(x) (where x is between about 0.05 and about 0.5), in addition to iron and/or nitrogen. In some examples, coating 18 may have a purity (e.g., collective iron and nitrogen content) of at least 92 atomic percent (at. %).

Coating 18 may include any selected thickness, and the thickness may at least partially depend on the CVD parameters, including the time for which the CVD technique is carried out.

In some examples, coating 18 additionally may include at least one dopant, such as a ferromagnetic or nonmagnetic dopant and/or a phase stabilizer. In some examples, at least one ferromagnetic or nonmagnetic dopant may be referred to as a ferromagnetic or nonmagnetic impurity and/or the phase stabilizer may be referred to as a phase stabilization impurity. A ferromagnetic or nonmagnetic dopant may be used to increase at least one of the magnetic moment, magnetic coercivity, or thermal stability of the hard magnetic material formed from coating 18. Examples of ferromagnetic or nonmagnetic dopants include Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Pt, Au, Sm, C, Pb, W, Ga, Y, Mg, Hf, and Ta. For example, including Mn dopant atoms at levels between about 5 at. % and about 15 at. % in an iron nitride material including at least one Fe₁₆N₂ phase domain may improve thermal stability of the Fe₁₆N₂ phase domains and magnetic coercivity of the material compared to an iron nitride material not including Mn dopant atoms. In some examples, more than one (e.g., at least two) ferromagnetic or nonmagnetic dopants may be includes in the mixture including iron and nitrogen. In some examples, the ferromagnetic or nonmagnetic dopants may function as domain wall pinning sites, which may improve coercivity of the magnetic material formed from coating 18. Table 1 includes example concentrations of ferromagnetic or nonmagnetic dopants within coating 18.

TABLE 1 Dopant Concentration (at. %) Sc 0.1-33 Ti 0.1-28 V 0.1-25 Nb 0.1-27 Cr 0.1-10 Mo 0.1-3  Mn 0.1-28 Ru  2-28 Co 0.1-50 Rh  11-48 Ni  2-71 Pd 0.1-55 Pt 0.1-15 Cu 0.1-30 Ag  1-10 Au  1-10 Zn 0.1-30 Cd 0.1-35 Zr 0.1-33 Pb 0.1-60 Mg 0.1-60 W 0.1-20 Ta 0.1-20 Ga 0.1-10 Sm 0.1-11

Alternatively or additionally, coating 18 may include at least one phase stabilizer. The at least one phase stabilizer may be an element selected to improve at least one of Fe₁₆N₂ volume ratio, thermal stability, coercivity, and erosion resistance. When present in coating 18, the at least one phase stabilizer may be present in the mixture including iron and nitrogen at a concentration between about 0.1 at. % and about 15 at. %. In some examples in which at least two phase stabilizers at present in the mixture, the total concentration of the at least two phase stabilizers may be between about 0.1 at. % and about 15 at. %. The at least one phase stabilizer may include, for example, B, Al, C, Si, P, O, Co, Cr, Mn, and/or S. For example, including Mn dopant atoms at levels between about 5 at. % and about 15 at. % in an iron nitride material including at least one Fe₁₆N₂ phase domain may improve thermal stability of the Fe₁₆N₂ phase domains and magnetic coercivity of the iron nitride material compared to an iron nitride material not including Mn dopant atoms.

Once coating 18 has been formed to a predetermined thickness, substrate 16 and coating 18 may be removed from CVD chamber 12 and subjected to an annealing technique. The annealing technique may facilitate formation of α″-Fe₁₆N₂ hard magnetic phase in coating 18.

The annealing technique may be carried out at a temperature that produces strain in coating 18 due to differences in the coefficients of thermal expansion for substrate 16 and coating 18 to access the α″-Fe₁₆N₂ phase. Additionally, the annealing technique allows diffusion of N+ ions within iron crystals in coating 18 to form iron nitride, including α″-Fe₁₆N₂ phase domains and Fe₈N phase domains. FIG. 2 is an iron nitride phase diagram, reproduced from E. H. Du Marchi Van Voorthuysen et al. Low-Temperature Extension of the Lehrer Diagram and the Iron-Nitrogen Phase Diagram, 33A Metallurgical and Materials Transactions A 2593, 2597 (August 2002), the entire content of which is incorporated herein by reference. As shown in FIG. 2, annealing at relatively low temperatures allows transformation of partial Fe₈N disordered phase into α″-Fe₁₆N₂ ordered phase.

In some examples, the annealing technique may be carried out at a temperature below about 250° C., such as between about 120° C. and about 220° C., between about 120° C. and about 200° C., between about 150° C. and about 200° C., or at about 150° C. The annealing technique may be performed in a nitrogen (N₂) or argon (Ar) atmosphere, or in a vacuum or near-vacuum.

The temperature and duration of the annealing step may be selected based on, for example, a size of the sample and a diffusion coefficient of nitrogen atoms in iron at the annealing temperature. Based on these factors, the temperature and duration may be selected to provide sufficient time for nitrogen atoms to diffuse to locations within coating 18 to form Fe₁₆N₂ domains.

Additionally, the temperature and duration of the annealing technique may be selected based on a desired volume fraction of α″-Fe₁₆N₂ phase domains in coating 18. For example, at a selected temperature, a longer annealing technique may result in a higher volume fraction of α″-Fe₁₆N₂. Similarly, for a given annealing technique duration, a higher temperature may result in a higher volume fraction of α″-Fe₁₆N₂. However, for durations above a threshold value, the additional volume fraction of α″-Fe₁₆N₂ may be limited or eliminated, as the volume fraction of α″-Fe₁₆N₂ reaches a relatively stable value. For example, at a temperature of about 150° C., after about 20 hours, the volume fraction of α″-Fe₁₆N₂ reaches a stable value. The duration of the annealing technique may be at least about 5 hours, such as at least about 20 hours, or between about 5 hours and about 100 hours, or between about 5 hours and about 80 hours, or between about 20 hours and about 80 hours, or about 40 hours.

Fe₈N and α″-Fe₁₆N₂ have similar body-centered tetragonal (bct) crystalline structure. However, in α″-Fe₁₆N₂, nitrogen atoms are ordered within the iron lattice, while in Fe₈N, nitrogen atoms are randomly distributed within the iron lattice. FIG. 3 is a conceptual diagram that shows an α″-Fe₁₆N₂ unit cell. As shown in FIG. 3, in the α″-Fe₁₆N₂ phase, the nitrogen atoms are aligned along the (002) (iron) crystal planes. Also shown in FIG. 3, the iron nitride unit cell is distorted such that the length of the unit cell along the <001> axis is approximately 6.28 angstroms (Å) while the length of the unit cell along the <010> and <100> axes is approximately 5.72 Å. The α″-Fe₁₆N₂ unit cell may be referred to as a bct unit cell when in the strained state. When the α″-Fe₁₆N₂ unit cell is in the strained state, the <001> axis may be referred to as the c-axis of the unit cell.

The annealing technique facilitates formation of the bct crystalline structure at least in part due to the strain exerted on the iron crystal lattice as a result of differential expansion of the substrate and the iron nitride workpiece during the post-annealing step. For example, the coefficient of thermal expansion for iron is 11.8 μm/m·K, while for silicon it is 2.6 μm/m·K. This difference in thermal expansion coefficients results in a compression stress substantially parallel the major plane of coating 18 and a corresponding stretching force being generated along the <001> crystalline direction on a coating 18 with an (110) face. In some examples, the strain on coating 18 may be between about 0.3% and about 7%, which may result in a substantially similar strain on individual crystals of the iron nitride, such that the unit cell is elongated along the <001> axis between about 0.3% and about 7%. This may facilitate incorporation of nitrogen atoms at the preferred positions of the α″-Fe₁₆N₂ crystal.

In some examples, rather than transforming all of coating 18 to α″-Fe₁₆N₂ phase, the annealing technique may result in formation of α″-Fe₁₆N₂ phase domains within domains of Fe, Fe₈N, and/or iron nitride compositions. FIG. 4 is a conceptual diagram illustrating a material having iron or iron nitride (e.g., Fe₈N) domains 42 and α″-Fe₁₆N₂ domains 44. Because coating 18 may be structured on a nanometer scale (e.g., the sizes of iron or iron nitride domains 42 and α″-Fe₁₆N₂ domains 44 are on the order of nanometers), magnetic coupling between the magnetically hard α″-Fe₁₆N₂ domains 44 and the magnetically soft iron or iron nitride domains 42 may occur substantially throughout coating 18. Because the α″-Fe₁₆N₂ and iron or iron nitride crystals have substantially similar crystalline structure, the material can be naturally crystallographically coherent, meaning having an aligned easy axis, which produces anisotropy. This may facilitate exchange-spring coupling through phase boundaries between α″-Fe₁₆N₂ domains 44 and iron or iron nitride domains 42.

Exchange-spring coupling may effectively improve the magnetic energy product of magnetic materials and provide magnetic properties for the bulk material similar to those of a bulk material formed of α″-Fe₁₆N₂. To achieve exchange-spring coupling throughout the volume of the magnetic material, the α″-Fe₁₆N₂ domains may be distributed throughout coating 18, e.g., at a nanometer or micrometer scale. For example, the magnetically hard Fe₁₆N₂ phase may constitute between about 5 vol. % and about 40 vol. % of the total volume of coating 18, or between about 5 vol. % and about 20 vol. % of the total volume of coating 18, or between about 10 vol. % and about 20 vol. % of the total volume of coating 18, or between about 10 vol. % and about 15 vol. % of the total volume of coating 18, or about 10 vol. % of the total volume of coating 18, with the remainder of the volume of coating 18 being magnetically soft materials. The magnetically soft materials may include, for example, Fe, FeCo, Fe₈N, or combinations thereof.

In some examples, such as when the magnetically soft material includes Fe or Fe₈N, the crystallographic texture of the Fe₁₆N₂ and the Fe or Fe₈N domains may be coherent. In other words, there may be a lattice match between the domains. This may facilitate efficient exchange-spring coupling between the magnetically hard Fe₁₆N₂ domains 77 and the magnetically soft iron or iron nitride domains 42, particularly across phase boundaries.

By using CVD to form coating 18 on substrate 16, hard magnetic material including α″-Fe₁₆N₂ may be incorporated into other products formed using CVD and existing manufacturing techniques that utilize CVD. Using existing CVD manufacturing operations, including masking, hard magnetic material including α″-Fe₁₆N₂ may be deposited on predetermined portions or regions of substrate 16. For example, hard magnetic materials including α″-Fe₁₆N₂ may be incorporated into CMOS (complementary metal-oxide-semiconductor) integrated circuit devices, and the CVD technique for forming hard magnetic materials including α″-Fe₁₆N₂ may be incorporated into existing CMOS processing techniques. For example, hard magnetic materials including α″-Fe₁₆N₂ may be incorporated into magnetic random access memory (MRAM), magnetic logic devices, magnetic storage devices, magnetic microelectromechanical systems (MEMS), micro motors, micro actuators, nano motors, nano actuators, or the like. In other examples, hard magnetic materials including α″-Fe₁₆N₂ formed using CVD may be incorporated into other devices utilizing hard magnetic materials, such as electric motors, electric generators, magnetic recording media, and magnetic resonance imaging (MRI) magnets, among other applications.

CVD may allow growth of coating 18 faster than some other techniques, such as molecular beam epitaxy (MBE), while, in some examples, forming superior coatings compared to some other techniques, such as sputtering.

Although FIG. 1 illustrates an example system 10 for CVD using a liquid iron-containing material, in other examples, CVD may be performed using a solid iron-containing material. FIG. 5 is a conceptual and schematic diagram illustrating an example chemical vapor deposition system 50 for forming a hard magnetic material including α″-Fe₁₆N₂. In some examples, system 50 of FIG. 5 may be similar to or substantially the same as system 10 described with reference to FIG. 1, aside from the differences described herein.

System 50 includes a CVD chamber 52. CVD chamber 52 encloses a susceptor 54, which may be similar or substantially the same as susceptor 14 of FIG. 1. In the example illustrated in FIG. 5, susceptor 54 is not shaped or oriented to position substrate 16 at an incline with respect to inlets 56 and 58. In other examples, susceptor 54 may be shaped or oriented to position substrate 16 at an incline with respect to inlets 56 and 58. CVD chamber 52 may include, for example, quartz or another refractory material. In some examples, CVD chamber 52 may be formed of a material that is substantially transparent to radio frequency (RF) magnetic energy.

CVD chamber 52 is at least partially surrounded by RF induction coils 20. RF induction coils 20 may be similar to or substantially the same as RF induction coils illustrated in FIG. 1. CVD chamber 52 encloses substrate 16, on which coating 18 is formed. Substrate 16 is disposed on susceptor 54.

In some examples, rather than including a susceptor 54 heated by RF induction coils 20, CVD chamber 52 may be heated such that an entire volume of CVD chamber 52 is heated. For example, CVD chamber 52 may be disposed in a furnace, or CVD chamber 52 may be formed of a material that absorbs RF energy and heats the volume of CVD chamber 52.

CVD chamber 52 may include inlet 56 and 58 and an outlet 24. Inlets 56 and 58 may be fluidically connected to one or more sources of coating gases. For example, in system 50, inlet 56 is fluidically connected to a chamber 60 enclosing a solid iron-containing material 62 and inlet 58 is fluidically coupled to first source 30 via a valve 32. First source 30 and valve 32 may be similar to or substantially the same as described above with respect to FIG. 1. For example, first source 30 may include a source of a vapor including a nitrogen-containing compound.

Chamber 60 encloses a solid iron-containing material 62. In some examples, iron-containing material 62 may include an iron-containing powder, billet, or thin film deposited on a substrate. In some examples, the particles in the iron-containing powder may define an average characteristic dimension of on the order of nanometers or micrometers. In some examples, the iron-containing thin film may define a thickness between about 500 nanometers (nm) and about 1 millimeter (mm). In some examples, iron-containing material 62 includes substantially pure iron (e.g., iron with a purity of greater than 90 at. %). In other examples, iron-containing material 62 may include iron oxide (e.g., Fe₂O₃ or Fe₃O₄).

Chamber 60 may include a first inlet 64 and a second inlet 66. First inlet 64 may fluidically connected to a first gas source 68 by a valve 70. First gas source 68 may include a source of an acid or chloride, such as HCl. The acid or chloride may react with iron-containing material 62 to form an iron-containing vapor. For example, HCl may react with iron-containing material 62 to form iron chloride (FeCl₃), which may be heated to form a vapor.

Second inlet 66 may be fluidically coupled to a carrier gas source 72 by a valve 74. In some examples, carrier gas source 72 may include a source of substantially inert gas (e.g., a gas that is substantially non-reactive with other elements and compounds present in system 50 during operation of system 50). A substantially inert gas may include, for example, a noble gas, such as argon.

Valves 32, 70, and 74 may be used to control the total flow rate of gases and vapors into CVD chamber 52, and the relative proportion of carrier gas, nitrogen-containing vapor, and iron-containing vapor in the gases and vapors flowing into CVD chamber 52. In some examples, valves 32, 70, and 74 may be controlled to produce an atomic ratio of iron to nitrogen in the gases and vapors flowing into CVD chamber 52 to be between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen). For example, the atomic ratio between iron and nitrogen atoms in the of the gases and vapors flowing into CVD chamber 52 may be about 9:1 (iron:nitrogen), about 8:1 (iron:nitrogen), or about 6.65:1 (iron:nitrogen).

In some examples, the flow rate of the carrier gas may be between about 100 sccm and about 5,000 sccm, the flow rate of the nitrogen-containing vapor may be between about 10 sccm and about 1,000 sccm, and the flow rate of the acid or chloride may be between about 10 sccm and about 1,000 sccm. Flow rates such as these may result in a growth rate of coating of between about 100 μm/h and about 1,000 μm/h.

In some examples, the HCl may react with Fe in chamber 60 according to the following reaction: Fe+HCl→FeCl₃+H₂ The FeCl₃ and H₂ may flow into CVD chamber 52 through first inlet 56, where the vapors may mix with the nitrogen-containing vapor, such as NH₃. In some examples, the nitrogen-containing vapor and the iron containing vapor may react according to the following reaction to deposit coating 18 including an approximately 8:1 ratio of iron to nitrogen: 16FeCl₃+2NH₃+21H₂→2Fe₈N+48HCl

As described above with respect to FIG. 1, once coating 18 has been formed to a predetermined thickness, coating 18 may be annealed to transform at least some of the iron nitride mixture in coating 18 to α″-Fe₁₆N₂. The annealing technique may be similar to or substantially the same as that described above with respect to FIG. 1.

In other examples, a coating (e.g., coating 18) may be formed on a substrate (e.g., substrate 16) using liquid phase epitaxy (LPE). In LPE, a solution including the coating materials may be cooled to form a supersaturated solution. The coating materials in the solution deposit a coating on a substrate immersed in the solution. In some examples, the degree of supersaturation may be low, such that the LPE technique is a near-equilibrium process. This may result in coatings with high crystalline quality (e.g., near-perfect crystalline structure). Additionally, because the concentration of the coating materials in the solution are much greater than the concentration of coating materials in vapor phase techniques, the growth rate of the coating may be greater than the growth rate for coatings grown using vapor phase techniques.

FIG. 6 is a conceptual and schematic diagram illustrating an example system 80 for forming a coating including α″-Fe₁₆N₂ on a substrate 82 using LPE. System 80 includes a crucible 82 in which a coating solution 86 is contained. System 80 also includes RF induction coils 84, which at least partially surrounded crucible 82. RF induction coils 84 may be electrically connected to an RF source (not shown in FIG. 6), which causes an alternating electrical current at RF to flow through RF induction coils 84. In some examples, the RF magnetic field generated by RF induction coils 84 may be absorbed by coating solution 86 or by crucible 82, such that coating solution 86 is heated.

Coating solution 86 may include a solution of iron in a solvent. In some examples, the solvent may include a nitrogen-containing compound, such as ammonium nitrate, urea, an amide or hydrazine. In some examples, the solvent may be oversaturated with nitrogen at the deposition temperature and pressure. Example amides include carbamide ((NH₂)₂CO; also referred to as urea), methanamide (Formula 1 above), benzamide (Formula 2 above), acetamide (Formula 3 above), and acid amides, although any amide may be used. The amide may be selected to be a liquid at the temperatures experienced by coating solution 86 during the LPE technique.

Coating solution 86 also includes an iron source. In some examples, the iron source may include an iron-containing compound. In some examples, the iron source includes a liquid iron donor, such as FeCl₃ or Fe(CO)₅. In other examples, the iron source may include an iron-containing powder. In some examples, the iron-containing powder may include substantially pure iron (e.g., iron with a purity of greater than 90 at. %). In some examples, the iron-containing powder may include iron oxide (e.g., Fe₂O₃ or Fe₃O₄).

During the LPE process, the coating solution 86 may be heated to a temperature above the liquidus temperature of the liquid-solid iron-nitrogen phase diagram shown in FIG. 7. For example, the solvent may be heated to the temperature indicated by point A in the liquid-solid iron-nitrogen phase diagram. In some examples, the solvent may not include the iron source when heated to the temperature indicated by point A.

The iron source or iron-containing material then may be dissolved in the solvent to form a coating solution 86 that is saturated with the iron-containing material. This solution saturated solution is indicated by point B on the liquid-solid iron-nitrogen phase diagram. Substrate 16 then may be immersed in coating solution 86.

Coating solution 86 and substrate 16 then may be cooled to a temperature, indicated by point C, that is below the liquidus. This causes coating solution 86 to be supersaturated with the iron-containing material, which drives the LPE coating technique. In some examples, the temperature indicated by point C, at which the LPE coating technique is performed, may be between about 600° C. and about 800° C. (although point C indicates a higher temperature). Point C is in the two-phase region, which provides a driving force for precipitation of iron nitride on the surface of substrate 16, until point D on the liquidus line is reached, where precipitation ends. In some examples, the concentration of iron and nitrogen in coating solution 86 and the temperature at which the LPE coating technique is performed may be controlled to provide an atomic ratio of iron to nitrogen between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen). For example, the atomic ratio between iron and nitrogen atoms may be about 9:1 (iron:nitrogen), about 8:1 (iron:nitrogen), or about 6.65:1 (iron:nitrogen).

Once the coating including iron and nitrogen is formed by LPE, substrate 16 may be removed from crucible 82, and the coating may be annealed under conditions similar to or substantially the same as those described with respect to FIG. 1. The annealing may facilitate formation of α″-Fe₁₆N₂ in the coating.

Because α″-Fe₁₆N₂ has high saturation magnetization and magnetic anisotropy constant. The high saturation magnetization and magnetic anisotropy constants result in a magnetic energy product that may be higher than rare earth magnets. For example, experimental evidence gathered from thin film α″-Fe₁₆N₂ permanent magnets suggests that bulk Fe₁₆N₂ permanent magnets may have desirable magnetic properties, including a magnetic energy product of as high as about 134 MegaGauss*Oerstads (MGOe), which is about two times the magnetic energy product of NdFeB (which has a magnetic energy product of about 60 MGOe). Additionally, iron and nitrogen are abundant elements, and thus are relatively inexpensive and easy to procure. The high magnetic energy product of α″-Fe₁₆N₂ magnets may be used in electric motors, electric generators, magnetic recording media, and magnetic resonance imaging (MRI) magnets, among other applications.

When ranges are used herein for physical properties, such as molecular weight, or chemical properties, such as chemical formulae, all combinations and subcombinations of ranges for specific examples therein are intended to be included.

Various examples have been described. Those skilled in the art will appreciate that numerous changes and modifications can be made to the examples described in this disclosure and that such changes and modifications can be made without departing from the spirit of the disclosure. These and other examples are within the scope of the following claims.

This patent application is related to International Patent Application No. PCT/US2012/051382, filed Aug. 17, 2012; International Patent Application No. PCT/US2014/015104, filed Feb. 6, 2014; and International Patent Application No. PCT/US2014/043902, filed Jun. 24, 2014. The entire contents of these international patent applications are incorporated herein by reference.

Clause 1: A method comprising: heating an iron source to form a vapor comprising an iron-containing compound; depositing iron from the vapor comprising the iron-containing compound and nitrogen from a vapor comprising a nitrogen-containing compound on a substrate to form a layer comprising iron and nitrogen; and annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂.

Clause 2: The method of clause 1, wherein the iron source comprises solid iron.

Clause 3: The method of clause 1, wherein the solid iron comprises at least one of iron powder or an iron thin film.

Clause 4: The method of clause 1, wherein iron source comprises a solid iron precursor.

Clause 5: The method of clause 4, wherein the solid iron precursor comprises at least one of Fe₂O₃ powder or Fe₂O₄ powder.

Clause 6: The method of clause 1, wherein the iron source comprises a liquid iron precursor.

Clause 7: The method of clause 6, wherein the liquid iron precursor comprises at least one of FeCl₃ or Fe(CO)₅.

Clause 8: The method of any one of clauses 1 to 7, wherein the vapor comprising the nitrogen-containing compound is formed by heating urea to form a urea vapor.

Clause 9: The method of any one of clauses 1 to 7, wherein the vapor comprising the nitrogen-containing compound is formed by heating at least one of an amide or hydrazine to form the vapor comprising nitrogen.

Clause 10: The method of any one of clauses 1 to 7, wherein the vapor comprising the nitrogen-containing compound comprises NH₃ vapor.

Clause 11: The method of any one of clauses 1 to 7, wherein the vapor comprising the nitrogen-containing compound comprises atomic nitrogen formed from diatomic nitrogen using a plasma.

Clause 12: The method of any one of clauses 1 to 11, further comprising heating the vapor comprising the iron-containing compound and the vapor comprising the nitrogen-containing compound to decompose the vapor comprising the iron-containing compound and the vapor comprising the nitrogen-containing compound to form atomic nitrogen and atomic iron, which deposit on the substrate.

Clause 13: The method of any one of clauses 1 to 11, further comprising heating the substrate above a decomposition temperature of at least one of the vapor comprising the iron-containing compound and the vapor comprising the nitrogen-containing compound.

Clause 14: The method of any one of clauses 1 to 13, wherein annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂ comprises heating the layer at a temperature between about 100° C. and about 220° C. for between about 5 hours and 80 hours.

Clause 15: The method of any one of clauses 1 to 14, wherein the substrate comprises at least one of silicon, GaAs, SiC, InGaAs, MgO, SiO₂, a high temperature polymer, or glass.

Clause 16: A method comprising: submerging a substrate in a coating solution comprising a nitrogen-containing solvent and an iron source, wherein the coating solution is saturated with the iron source at a first temperature above a liquidus temperature of an iron-nitrogen mixture to be deposited from the coating solution; cooling the coating solution to a second temperature to form a supersaturated coating solution, wherein the second temperature is below the liquidus temperature of the iron-nitrogen mixture; keeping the substrate in the supersaturated coating solution to allow a coating comprising iron and nitrogen to form on the substrate; and annealing the coating comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂.

Clause 17: The method of clause 16, wherein the solvent comprises at least one of ammonium nitrate, an amide, or hydrazine.

Clause 18: The method of clause 16 or 17, wherein the iron source comprises at least one of substantially pure iron, FeCl₃, Fe(CO)₅, or an iron oxide.

Clause 19: The method of any one of clauses 16 to 18, wherein the second temperature is between about 600° C. and about 800° C.

Clause 20: The method of any one of clauses 16 to 19, wherein the coating comprising iron and nitrogen comprises an atomic ratio of iron to nitrogen between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen).

Clause 21: The method of any one of clauses 16 to 19, wherein the coating comprising iron and nitrogen comprises an atomic ratio of iron to nitrogen about 8:1 (iron:nitrogen).

Clause 22: The method of any one of clauses 16 to 21, wherein annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂ comprises heating the layer at a temperature between about 100° C. and about 220° C. for between about 5 hours and 80 hours.

Clause 23: The method of any one of clauses 16 to 22, wherein the substrate comprises at least one of silicon, GaAs, SiC, InGaAs, MgO, SiO₂, a high temperature polymer, or glass.

Clause 24: An article formed by the method of any one of clauses 1 to 23.

Clause 25: A system for performing the method of any one of clauses 1 to 23.

The disclosure of each patent, patent application, and publication cited or described in this document are hereby incorporated herein by reference, in its entirety. 

What is claimed is:
 1. A method comprising: submerging a substrate in a coating solution comprising a nitrogen-containing solvent and an iron source, wherein the coating solution is saturated with the iron source at a first temperature above a liquidus temperature of an iron-nitrogen mixture to be deposited from the coating solution; cooling the coating solution to a second temperature to form a supersaturated coating solution, wherein the second temperature is below the liquidus temperature of the iron-nitrogen mixture; keeping the substrate in the supersaturated coating solution to allow a coating comprising iron and nitrogen to form on the substrate; and annealing the coating comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂.
 2. The method of claim 1, wherein the solvent comprises at least one of ammonium nitrate, an amide, or hydrazine.
 3. The method of claim 1, wherein the iron source comprises at least one of substantially pure iron, FeCl₃, Fe(CO)₅, or an iron oxide.
 4. The method of claim 1, wherein the second temperature is between about 600° C. and about 800° C.
 5. The method of claim 1, wherein the coating comprising iron and nitrogen comprises an atomic ratio of iron to nitrogen between about 11.5:1 (iron:nitrogen) and about 5.65:1 (iron:nitrogen).
 6. The method of claim 1, wherein the coating comprising iron and nitrogen comprises an atomic ratio of iron to nitrogen about 8:1 (iron:nitrogen).
 7. The method of claim 1, wherein annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe₁₆N₂ comprises heating the layer at a temperature between about 100° C. and about 220° C. for between about 5 hours and 80 hours. 